BGP Prefix-Lists

Prefix lists are more sophisticated forms that Cisco provides for filtering
BGP route advertisements. They filter on IP address just as distribute-lists
do, however they are easier to read, and require fewer commands to configure.
The other advantage to a distribute list is that it is easeir to add, remove
and organize the statements in the manner you chose.

While this configuration requires the same number of statements as the distribute
list example, you have the option of adding ge, or le to make
statements more flexible as to how you will permit blocks in that range.

For example:

prefix-list xx seq 10 permit 63.1.0.0/16 ge 18

The statement above allows any route announcement in the range of 63.1.0.0
- 63.1.255.255 but that announcement must have a length greater than 18 bits
in the mask. This permits you to allow announcements in the range, but not
an announcement equalling the entire range (/16), or even announcements of
half the range (/17). Only announcments with a length "greater than or
equal to" /18 will be permitted.